How to Sheath a Flat Roof

Installing sheathing on a flat roof is much easier than working on a peaked roof. The sheathing is the base you lay over the rafters that holds the roofing felt and shingles. Plywood is typically used as sheathing, although you can use particle board or oriented-strand board. Although the process of laying sheathing isn't complicated, be prepared for some hard work as you climb up and down a ladder and lift sheets of plywood.

1

Calculate the amount of sheathing you need. Measure the dimensions of your roof, then multiply that number together to the the square footage. Divide the square footage by 32, which is the square footage of a standard 4-by-8-foot sheet of plywood. That gives you the number of plywood sheets you need, although it's best to buy at least 10 percent more than you think you need to account for accidents or mistaken cuts.

2

Lay a long piece of lumber across your roof to make sure the rafters are level. Hammer shims under the rafters where they connect to the building frame if necessary to make them level.

3

Place the first piece of plywood on the bottom corner of the roof, lining it up with the edge of the rafters. Lay the plywood with its long edge perpendicular to the rafters to create the strongest roof base.

4

Snap a chalk line down the line of the rafters on the piece of plywood so you can find the rafters with your nail gun. Nail the plywood around the top, outside and bottom edges every 6 inches on the short side and 12 inches on the long sides using a nail gun filled with 8d nails. The nails should fall about 3/8 inch inside the edge of the plywood.

5

Slide 1/8-inch plastic plywood spacer clips onto the sides of the plywood that will touch other pieces of plywood. These spacers create expansion gaps to keep the sheathing from buckling in humid weather.

6

Add the next piece of plywood beside the first one, sliding it into the other end of the spacer clips. Verify that the plywood pieces align in the center of a rafter. If not, push the rafter over slightly until it lines up in the center of the plywood seam, then nail both pieces of plywood to the rafter. Nail the top and bottom of the second piece of plywood, then add spacers.

7

Cut the last piece in the row to fit using a circular saw. Nail all four sides in place.

8

Cut a piece of plywood in half so it measures 4 feet by 4 feet. Start the second row with it above the first panel you installed, adjust size if necessary to make it end in the center of a rafter. Cutting the first piece at the beginning of every other row alternates the plywood seams, which helps strengthen and waterproof your roof. Nail it on three sides and slide on spacer clips.

9

Finish the row using the same process as the first row, then continue adding rows until the roof is covered. Cut the last row lengthwise to fit using a circular saw.

Things You Will Need

Measuring tape

Long lumber piece

4-by-8-foot plywood panels, 5/8 inches thick

Chalk line

Nail gun

8d nails

1/8-inch plastic plywood spacer clips

Circular saw

Tips

Check your local building codes before installing the sheathing. Some require different thicknesses of plywood or a specific nail spacing.

Having a helper available to hand you the plywood panels can speed up the process significantly.

Warning

Stand on a ladder when installing the first panel. Only stand on panels that have been secured to the rafters with nails.

About the Author

Based outside Atlanta, Ga., Shala Munroe has been writing and copy editing since 1995. Beginning her career at newspapers such as the "Marietta Daily Journal" and the "Atlanta Business Chronicle," she most recently worked in communications and management for several nonprofit organizations before purchasing a flower shop in 2006. She earned a BA in communications from Jacksonville State University.